feat: expose catalog as Colormap classmethod

[tlambert03]

This PR is working towards making it easier to query the available colormap keys. It adds a couple of public conveniences:

from cmap import Colormap

# access global catalog
catalog = Colormap.catalog()

# various ways to query for available keys:


def unique_keys(
    self, prefer_short_names: bool = True, normalized_names: bool = False
) -> set[str]:
    """Return names that refer to unique colormap data.

    Parameters
    ----------
    prefer_short_names : bool, optional
        If True (default), short names (without the namespace prefix) will be
        preferred over fully qualified names. In cases where the same short name
        is used in multiple namespaces, they will *all* be referred to by their
        fully qualified (namespaced) name.
    normalized_names : bool, optional
        If True, return the normalized names of the colormaps.  If False
        (default), return the original names of the colormaps (which may include
        spaces and/or capital letters).
    """

def short_keys(self) -> set[str]:
    """Return a set of available short colormap names, without namespace."""

def namespaced_keys(self) -> set[str]:
    """Return a set of available short colormap names, with namespace."""

def resolve(self, name: str) -> str:
    """Return the fully qualified, normalized name of a colormap or alias."""

[codecov]

Codecov Report

Merging #22 (0d6e074) into main (c089822) will increase coverage by 0.02%.
The diff coverage is 98.91%.

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   96.88%   96.90%   +0.02%     
==========================================
  Files         151      151              
  Lines        1796     1842      +46     
==========================================
+ Hits         1740     1785      +45     
- Misses         56       57       +1     

Files Changed	Coverage Δ
src/cmap/_external.py	94.91% <ø> (ø)
src/cmap/_catalog.py	99.19% <98.75%> (-0.81%)	⬇️
src/cmap/__init__.py	100.00% <100.00%> (ø)
src/cmap/_colormap.py	87.98% <100.00%> (+0.19%)	⬆️

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